The present invention relates to a control method of phase modifying equipment for a DC power transmission system or for a frequency conversion system, and to a control apparatus utilizing this control method.
A conventional DC power transmission system is formed with a forward converter (rectifier) which converts AC power from an AC line into DC power, a DC power transmission line which transfers the converted DC power to another place, and a reverse converter (inverter) located at another place, which converts the transmitted DC power into another AC power and supplies the converted AC power to another AC line.
Such a DC power transmission system has an operation characteristic as shown in FIG. 1. In FIG. 1, the abscissa denotes a DC current Id of the DC power transmission line and the ordinate denotes a DC voltage ED thereof.
Referring to FIG. 1, portions (a), (b) and (c) are an operation curve of a rectifier. Portions (a) and (b) define a regulation characteristic of the rectifier. Portions (b) and (c) define a constant current characteristic of the rectifier. Portions (d), (e) and (f) are an operation curve of an inverter. Portions (d) and (e) define a constant current characteristic of the inverter. Portions (e) and (f) define a constant margin angle characteristic of the inverter. The difference between DC currents at portions (c) and (d) represents a current margin of the DC power transmission system.
The combination of the above rectifier and inverter is operated at a point X in FIG. 1. Point X is defined by the intersection between the operation curves of the rectifier and inverter. As may be seen from the operation curves in FIG. 1, the DC power transmission system is controlled such that the inverter determines DC voltage Ed of the power transmission line while the rectifier determines DC current Id thereof.
Both of the above rectifier and inverter serve as a phase-delayed load for the AC line. Also, the power factor of the rectifier and inverter is substantially proportional to the cosine of a control angle.
Assume that a reference margin angle which determines the constant margin angle characteristic of the inverter is enhanced to increase the phase-delayed reactive power of the inverter, and both the rectifier and inverter are operated at point X in FIG. 1.
In this case, DC voltage Ed is decreased, and the operation curve of the inverter shifts from portions (d), (e) and (f) to portions (dd), (ee) and (ff). Meanwhile, DC current Id is increased to compensate the decrease in DC voltage Ed so that a constant power transmission is achieved. Then, the operation curve of the rectifier is shifted from portions (a), (b) and (c) to portions (a), (bb) and (cc), and the operating point of the rectifier/inverter is shifted from point X to a point XX. (Since the transmission power is defined by the a product of DC voltage Ed and DC current Id, the curve of constant power becomes hyperbolic and the operating point of the rectifier/inverter is fixed on such a hyperbolic curve HC, as shown in FIG. 1).
Conventionally, AC lines coupled to the DC power transmission system are provided with shunt reactors and shunt capacitors. The circuit connection of these shunt reactors and shunt capacitors is controlled according to the value of transmission power. For instance, when the transmission power is 30% or less of a rated power (100% power output), only the shunt reactor is connected to the AC line. When the transmission power falls within a power range of 30% to 70% of the rated power, both of the shunt reactor and shunt capacitor are disconnected from the AC line. Within a power range of 70% or more, only the shunt capacitor is connected to the AC line.
The above connection control of shunt reactor/shunt capacitor is sufficient in a case where a DC voltage control or margin angle control is effected without performing a reactive power control or AC voltage control of the AC line. However, if a reactive power control is performed together with the DC voltage control or margin angle control, a certain problem has occurred. Such a problem will be discussed below.
FIG. 2 illustrates a reactive power controllable range of a conventional DC power transmission system. In FIG. 2, the abscissa denotes transmission power (detected active power) Pd and the ordinate denotes reactive power Q. The reference symbol "p.u" denotes a reference power (power unit) representing the 100% power output. In the following, for the sake of simplicity, it is assumed that shunt reactors have the same reactive power capacities as those of shunt capacitors. Further, AC filters being provided to eliminate high-frequency components from rectifier/inverter are considered as reactive power sources. The illustration of FIG. 2 contains the reactive power capacities of such AC filters.
Referring to FIG. 2, within the hatched region surrounded by points A, B, C, D and E, only a shunt capacitor is connected to the AC line of the inverter. Curve A - B indicates an active power-reactive power (P-Q) relationshp which is obtained by the system operation in accordance with a minimum margin angle .gamma.min. Curve D - E indicates another P-Q relationshp which is obtained by the system operation in accordance with a maximum margin angle .gamma.max and represents continuous operation at a rated current (100% current output) of the rectifier. Lines A - E and B - C respectively indicate the lower and upper limits of transmission power. Curve C - D indicates the limit of continuous operation with the rated (100%) current.
When a shunt capacitor is disconnected from the AC line of the inverter, the hatched region in FIG. 2 is parallel-shifted downward. When a shunt reactor is connected to the AC line, the above shifted region is further shifted downward in parallel to the former shift. Thus, the phase modifying control is effected by the selective connection of the shunt capacitor and/or shunt reactor. The reactive power controllable region in which the shunt reactor is connected and the shunt capacitor is disconnected is surrounded by points G, H, I, J and K.
According to a prior art control method as mentioned above, the shunt capacitor of the inverter side is connected at, e.g., a point DD in FIG. 2. Point DD corresponds to about 70% of the reference power "p.u". In this case, as may be seen from FIG. 2, the rectifier/inverter cannot operate in a region surrounded by points A, BB, CC, F and E. This means that the reactive power controllable region of the DC power transmission system is considerably narrowed by the operation of the above phase modifying control. This is a problem to be solved by the present invention. (A similar problem will be involved when the inverter is used as a frequency converter.)